It reads like the contributor was trying to pre-empt any fear of the work's age on the part of prospective readers rather than warning of its shortcomings on that account, the author's political leanings apparently being the greater concern. There does however seem to be a modern preference (perhaps over-represented online) for newer texts, which I consider largely misplaced as I said here.

That said, I do think understanding of the past has expanded so phenomenally in the past 70 years (I'd date the onset of the transformation to academic growth, incorporation of a wider range of historical experiences and the rise of new modes of analysis and interpretation from c.1950) that it's generally essential to supplement reading of older works with more recent findings unless there really aren't any, in which case it may offer a promising area for fresh research.

That doesn't mean there's anything inherently "wrong" with a book from 1965 or even 1935, it's just necessarily not updated for intervening scholarship, but it may nevertheless remain essential reading in its own right: nor are new finds always as groundbreaking as they purport to be: I'm regularly astonished to read of a recent discovery that I'd encountered in dusty tomes years ago.